1. Field of the Invention
The present invention relates to composite materials comprising at least one styrenic polymer, as defined herein, e.g., polystyrene or polymers completely miscible with polystyrene, and minerals, and/or an organic filler, and/or glass fiber, and/or thermoplastics.
2. Description of Related Art
For the past two decades very few new thermoplastic polymers have been developed. Instead, new materials have been created as alloys, blends, and composites of existing thermoplastic polymer materials, either in combination with each other or with inorganic reinforcing materials, such as minerals or glass fiber.
Generally, thermoplastics are divided into two groups. The first group comprises simple hydrocarbon polymers, e.g., polyolefins and styrenics typified by polypropylene and polystyrene. They have relatively modest physical properties, such as heat distortion, impact resistance, and the like, but are relatively simple to manufacture, easy to process, and have no reactive functional groups, and are thus resistant to humidity, aggressive chemicals, and the like.
The second group of thermoplastics, the so-called engineering resins, owe their superior physical properties, such as high strength and high heat stability, to the presence of highly polarized functional groups whose strong dipolar interactions lead to crystallinity and high melting points, but make them difficult to process and susceptible to hydrolysis and chemical degradation. Polyamides (Nylons), inter alia, typify this class of materials.
Thus, it has been an objective in polymer development to take advantage of the ease of manufacture, chemical stability, and ease of processing of the simple polyolefin and styrenic polymers, while imbuing them with improved physical properties by creating composite materials that are alloys or blends of them with glass and/or other reinforcing minerals and/or with polymers representing the engineering resins. Unfortunately, because the simple hydrocarbon polymers of the first type are non-polar, and engineering resins, minerals, and glass fibers are highly polar, it is not possible to obtain composites having desirable properties simply by blending the desired materials together. What results are materials exhibiting poor physical properties owing to irregular macro-phase separation and poor adhesion between the phases.
In order to solve this problem an additional component must be added, typically referred to as a compatibilizer, which must exhibit two properties to be successful. Analogous to a detergent dispersing oil in water, the compatibilizer must reduce the surface energy between the polar and non-polar phases of the composite, but it must also link the two phases of the composite together in order to give strength to the composite. Thus, a successful compatibilizer must have a polymeric architecture that provides two distinct domains. One domain must be compatible with the non-polar polymer material of the composite, and the other domain must be compatible with the polar phase of the composite, and, additionally, the non-polar domain of the compatibilizer must be sufficiently long to entangle its chains with the chains of the non-polar phase of the composite, and the polar domain of the compatibilizer must be either reactive with nucleophilic functional groups present in the polar phase of the composite so as to chemically bond it to that phase, or the polar domain of the compatibilizer must be sufficiently long to entangle its chains into the polar phase of the composite. Such compatibilizers allow both the formation of a uniform micro-phase dispersion of one composite component into the other and a strong linkage between the two phases.
The successful development of compatibilizers that permit composites of polyolefins (e.g., polypropylene) and minerals, glass, and/or polar thermoplastics, having excellent physical properties was rapid. For example, by the early 1970's compatibilizers based on maleated polypropylene were available for the manufacture of polyolefin based composite materials. Such compatibilizers are commercially available, such as those sold by Crompton Corporation under the trademark Polybond. The maleic anhydride domain of these compatibilizers is reacted with the nucleophilic amines and hydroxy functional groups in polyamides, polyesters, and polycarbonates, and with the amino silanes used to modify the surface of glass fibers and other mineral fillers. The architecture of maleated polypropylene is not random, but, rather, the maleic anhydride graft occurs naturally on the ends of the polypropylene chain. Thus, it meets both criteria for a successful compatibilizer: the maleated ends are reactive with and compatible with the polar phase of the composite, and the polyolefin domain is compatible with and sufficiently long to chain entangle with the polyolefin phase of the composite material.
The addition of two or three percent of such compatibilizer to an alloy of polypropylene with glass or polar thermoplastic polymer yields composites with fine dispersion and excellent physical properties. The same composites without the compatibilizer exhibit non-uniform macro-phase separation and poor physical properties.
Attempts to apply the analogous solution to the other major hydrocarbon polymer group, styrenics, have been without success. Maleation of polystyrene is random along the polystyrene chain and is not located on the ends of the chain, as is the case for polypropylene. Similarly, copolymerization of styrene monomer and maleic anhydride yields an alternating copolymer, and copolymerization of styrene with other nucleophile reactive monomers is random along the polystyrene chain.
Such candidate compatibilizers contain functional groups that are reactive with the nucleophiles present in the polar thermoplastics and amine modified fillers and therefore interact with the polar phase of the composites (e.g., glass, minerals, and or polar thermoplastic polymers), yielding in some cases more uniform dispersions of the one material in the other. However, because the architecture of these candidate compatibilizers is random, there are no separate domains, and, therefore, no domain that is compatible with the polystyrene phase of the composite and sufficiently long to chain entangle with the polystyrene in the composite. As a result, even with improved dispersion of one phase in the other, the required improvement in the physical properties of the alloy material is not achieved, and, indeed, sometimes there is even a degradation of physical properties compared to the same alloy without the candidate compatibilizer.
This success of the strategy with polyolefin composites and failure in polystyrene composites was studied and reported by Ide et al., J. Appl. Polym. Sci., 18(4):963–74 (1974), and the poor results in polystyrene composites using this strategy with a wide range of reactive monomer copolymerized with styrene or grafted randomly onto styrene has been widely reported throughout the polymer literature. See, for example, Park et al., Polymer, 37(14):3055–3063 (1996), Chang et al., Polymer Engineering and Science, 31(21): 1509–1519 (1991), and Jannasch et al., Journal of Applied Polymer Science, 58:753–770 (1995).
Subsequently, researchers recognized that the presence of reactive functional groups like maleic anhydride were necessary, but not sufficient for good compatibilization of polystyrene with polar materials. In addition to this, the placement of the nucleophile reactive functional groups within the compatibilizer polymer architecture must be non-random. With this understanding, better, but still insufficient, solutions were proposed. These solutions met both the criteria of dispersing one phase in the other and of creating links between the two phases in the composite, but at the price of complex methods of manufacture and, more seriously, the introduction into the polystyrene composite of extraneous materials that altered and partially degraded the intended physical and chemical property advantages of the alloy.
A maleate-grafted derivative of the well-known anionically produced tri-block copolymer polystyrene-polybutadiene-polystyrene is commercially available from Shell Chemical under the trade designation Kraton FX1901. Here, because polybutadiene is selectively maleated over polystyrene, only the polybutadiene block is maleated. This compatibilizer then meets both criteria. The center maleated butadiene block is reactive with the polar phase of the alloy, and the styrene blocks are compatible with and sufficiently long to chain entangle the polystyrene phase of the alloy. This compatibilizer has found application in certain cases wherein the introduction of a third rubbery phase is feasible. See, for example, Park et al., Polymer, 42:7465–7475 (2002). However, this compatibilizer's introduction to the alloy of an additional rubbery polybutadiene phase with low Tg and very poor oxidative stability is generally undesirable. The latter deficiency can be reduced in part by hydrogenation of the unsaturation in the polybutadiene, but this involves extra steps and still incorporates an extraneous polymer with lower physical properties into the composite.
Braat et al., WO 0046274 A1, observed that polyphenyleneoxide (PPO), which is thermodynamically compatible with styrene and has terminal phenol groups, may be maleated to produce PPO with terminal succinic anhydride groups. This compatibilizer meets both criteria for a successful compatibilizer in that the terminal succinic anhydride groups are reactive with the nucleophiles present in the polar phase of the composite and the PPO segment is compatible with the polystyrene phase of the composite and sufficiently long to chain entangle it. See, for example, Bank et al. WO 059197 A2. However, this compatibilizer also necessitates the introduction into the composite of an extraneous polymer (PPO) with different chemical and physical properties. For example, phenolics are well known to degrade in sunlight and oxygen to form highly colored quinones.
Koulouri et al., Macromolecules, 32:6242–6248 (1999) overcame the need to introduce an extraneous polymer into the alloy by carrying out atom-transfer-addition of maleic anhydride to the ends of alpha,omega-dibromopolystyrene. The resulting polystyrene is terminated by bromosuccinic anhydride. This compatibilizer is only polystyrene bearing nucleophile reactive end groups and the material was demonstrated to form excellent alloys with polyamides. Both dispersion and physical properties were significantly improved. However, the presence of bromosuccinic anhydride makes this compatibilizer impractical to use owing to the unstable carbon/bromine bond present in the bromosuccinic anhydride end groups, which is well known to release corrosive hydrobromic acid under mild conditions of for example, heat and/or humidity.
Thus, a need remains in the art for a means to produce an alloy between styrenic polymers and polar materials, such as glass, minerals, or polar thermoplastics, having both good phase separation/dispersion and good physical properties without the simultaneous inclusion of corrosive functional groups or extraneous polymeric materials with undesirable properties.